Let's talk about how to write chemical formulas
for ionic compounds. So what this means is we're going to start
the chemical name like Magnesium Chloride and learn the steps that we have to go through
to take this chemical name and use it to write a chemical formula like MgCl2, okay? Let's start with the first example, Lithium
Oxide. So when I'm doing these kind of problems,
the first thing that I want to do is find both these elements on the periodic table. I'm using this kind of weird version of the
periodic table that I just wrote out. I left out a lot of the elements because they're
not important for what we're doing here and I thought that they're kind of distracting
but don't be confused. This really is no different from periodic
table you probably have in your book, it's just that it's missing a lot of the elements. Anyway, Lithium . . . where do I find that? It's right here, Li, and Oxide. Oxide is just another word for Oxygen, it's
what we call Oxygen when Oxygen has a charge and I'm going to talk a little bit more about
that later. Anyway, Oxide is just another word for Oxygen
and Oxygen is over here. They're on opposite sides of the periodic
table and check this out too. There's this big thick staircase that separates
Lithium from Oxygen. What's a staircase doing? If you remember, the staircase separates the
metals on this side of the periodic table from the nonmetals on this side of the periodic
table. So Lithium's a metal and Oxygen is a nonmetal. This is important because we have a metal
and a nonmetal connected together and that means that we're dealing with an ionic compound. Ionic compounds are always metals and nonmetals. So we have metals and nonmetals making an
ionic compound, that means that the atoms in that compound have a charge, so I want
to find out what the charge of those atoms is and I can do that by looking at where they
live, what column they live in on the periodic table. Here's what I mean. So Lithium lives within this column. Everything in this column has a plus one charge,
so I'm going to write this right here . . . Li+1 . . . everything in this column has a two
plus charge (+2), three plus charge (+3), you may want to write this on your own periodic
table that you have for your reference. Everything here has a 3 minus charge (-3)
and Oxygen which lives in this column has a two minus charge (-2) so I'm going to write
it right here. So Li+1, O2-. Okay, so now I ask myself, does the plus charge
(+) and the minus charge (-) balance when I have one atom of both of these? The answer is no because I have one plus charge
but I have two minus charges so the charges don't balance. But I want to figure out how to balance them. I need to have them balanced in order to write
the chemical formula, so what I can do is I can add more Lithium atoms, I can add more
Oxygen atoms, or I can add both of them until I get the charges to balance out. I have two minus (2-) here and only one plus
(+1) here so what I'm going to do is I'm going to add another Lithium atom, Li+, so that
now I have two positive to balance out my two negative and now they balance. So in order to get the charges balanced, I
have to have two Lithiums and one Oxide or one Oxygen. Now when I write the chemical formula, I'm
literally going to say how many of each of these atoms I need in order for the charges
to balance. We've said that it's two Lithium atoms so
I'm going to write "Li" and then the two after it that indicates that I have two lithium
atoms and then one "O" to show that I have one Oxygen and I'm not going to write anything
after that. If you have a letter without anything after
it, it means that you just have one of them. So Lithium Oxide, two Lithiums and one oxygen,
is how we get the charges to balance for this compound. Here's the next one, Potassium Nitride. Just as before, I'm going to take out the
periodic table and I'm going to find both of these guys on it. Potassium is K, it's right over here and Nitride
is what we call Nitrogen when it has a charge on it and so Nitrogen is over here. Check it out! Metal, non-metal, separated by this big thick
staircase and so we know we're dealing with an ionic compound and whenever we think about
ionic compound . . . metals and nonmetals . . . we want to think charges, what charges
does the atoms have? Potassium lives in this column here, the same
column as Lithium and so it has a one plus charge (+1) so I'm going to write that up
here, K+. And Nitrogen lives in this column where everything
has a minus three charge (-3) so here we have N3-. When I have one atom of K+ and one atom of
N3- do the charges balance out here? Doe the plus and minus balance out? It doesn't, I got one plus and I've got three
minus, that doesn't balance right. But I could add more atoms of either type
in order to get the charges to balance. So since I don't have as much pluses as I
have minuses I'm going to add a bunch of more Potassium. Here I add another, now I have 2+, and now
I'm going to add one more and I have 3+ . . . 3+ Potassium on this side balances out the 3-
Nitride on this side. Now when I write my chemical formula, I want
to literally say how many atoms of each type I need for the charges to balance. I'm going to need three Potassiums so I'm
going to write "K3" and I only need one nitrogen or one Nitride so I'm going to write "N" with
nothing after it and if I don't write anything after, it means one. So three Potassiums, one Nitrogen, the correct
formula for this is K3N. Okay, here's another one. Sodium Chloride which is the name of common
table salt. Once again, I'm going to find them both on
the periodic table. Sodium right here is in the one plus (+1)
column and Chlorine or Chloride is in this column right here. We've got metal and nonmetal so we're dealing
with ionic compounds which means the atoms have charges. Now I want to figure out what the charges
are. As I said before, Na is in the one plus (+1)
so an atom of Sodium has a one plus (+1) charge here and then Chloride is what we call Chlorine
when it has a charge and Chlorine lives in this column so it has a one minus charge (-1)
so Cl-. Okay, so I have one atom of Sodium plus one
(Na+) and I have one atom of Chloride minus one (Cl-). When I have an atom of both of these, does
the positive and negative balance out? It does, I don't need to add more of either
one of these atoms, I'm totally set. So to write the chemical formula for Sodium
Chloride I want to say again how many of each I need for the charges to balance out. I'm just going to say NaCl. I don't put any numbers after them because
if I don't put a number it means I just have one. So I have one Sodium and one Chloride, okay? You might be getting the hang of this. I want to try to convince you to watch this
one last example because it's a tricky one that confuses a lot of people. Aluminum Oxide. Once again, periodic table. Aluminum here, Oxygen here, metal, non-metal,
it's ionic which means charges are important. So Aluminum is in the three plus (+3) column,
one atom or ion of Aluminum, an ion is what we call an atom that has a charge. An ion of Aluminum has a three plus charge
(+3) and an atom of Oxygen or oxide here has a two minus charge (-2), Al2+ O2-.Now I need
to figure out how many of each of these atoms I need for the charges to balance out and
this example is a little bit trickier than the ones before. How are we going to be able to add atoms to
get the same number of positive and negative charge? Let me give you a hint. We're going to start by adding another Aluminum
atom in here or Aluminum ion to be precise. So now I have three plus here and three-plus
here, each Aluminum atom has a charge of three plus (+3) so I have a total of six plus (+6). Now I can balance out the six plus by adding
more of the Oxygen. I add one more Oxygen and I have a total of
four minus and I add one more I have a total of six minus. So this is how we balance out the charges
in Aluminum Oxide with two Aluminum ions with three plus (+3) each, total of six plus (+6),
and three Oxygen ions with a total of two minus (-2) each which gives us minus six (-6). Now I'm going to write the chemical formula
to show how many each I need to have. That's going to be Al2 and O3. I have two Aluminums and three Oxygens. Okay, so now let me tell you what I'm going
to do. If you're still a little bit shaky on this,
I'm going to do a few more practice problems in a minute. If you feel really good about this, you might
want to just turn the video off and go and watch videos I have on writing formulas for
compounds with transition metals and for compounds with polyatomic ions. But before I do a few more practice problems
I want to talk about two commonly made mistakes so many people make when they're writing these
formulas so you might just want to stick around to watch that. Here are the two mistakes people make. Let's assume that you're asked to write the
chemical formula for Lithium Oxide, you do this you find out you want to have two Lithiums
with one plus each to balance out the charge on Oxygen, okay? Here is one mistake that people make. They say, okay, I have two Lithiums and one
Oxygen so I'll write LiO1. They say yeah, yeah, yeah there's only one
Oxygen so it can't hurt to just add the O1 here, can it? It can't hurt. It does hurt! It's just not right to do, you never want
to put a one after an element. So Li2O1 is wrong. Instead, you want to write Li2O without any
numbers after this and this is the right way to do it. Here's another mistake that people often do. They don't remember to get rid of the charges
when they write the chemical formula. So they end up writing something like Li+2O2-
where the charges that were on the atoms here stay when they write the chemical formula. This also unfortunately is wrong. When you write the chemical formula you want
to get rid of all of the charges so you just write again Li2O. Make sure there aren't any charges in the
chemical formula when you finally right it, okay?